Amplifier particularly operational amplifiers are as a rule constructed in several stages; in addition to a differential input stage and an output stage or end stage, further amplifier stages are optionally provided between these two stages. Whereas the input stage is substantially used for setting the amplification, the output stage primarily provides current to the load, which can fluctuate over a wide range depending on the application of the amplifier. In addition, it is desirable that the output stage should have low distortion and a low output resistance, and should not impair the amplification properties of the amplifier stage.
To ensure a large output voltage rise, and to improve the frequency response of the amplifier, so-called all-n-p-n push-pull output stages are used, in which n-p-n transistors are exclusively responsible for providing of the load current. In the quiescent state of the amplifier (no load current), a low (cross) current flows through the output transistors; the potential at the output of the output stage adjusts to a quiescent potential or balanced potential corresponding to approximately half the supply voltage. Under load, an output transistor should provide the current required to raise the output potential (positive half-wave), and in the case of a drop in the output potential another transistor should pick up the necessary current (negative half-wave). This is achieved by the transistors being triggered in complementary fashion by respective push and pull signals; the necessary amplifier circuits however have greatly differing amplification properties. The output transistors alternate depending on the polarity of the output voltage in the load current carried, with the possibility existing here of the output transistors being briefly without current during a change from the positive to the negative half-wave of the output voltage (non-ideal current transfer), hence causing a gap in the transmission or transfer characteristic. The quiescent (cross) current mentioned does avoid such gaps during current transfer, but the widely differing amplification properties of the output transistors cause major non-linearities in the transmission characteristic, which, particularly in the range of the balanced state, lead to a sharp kink in the characteristic; as a result, however, the transmission characteristics of the amplifier too, particularly at low output voltages, are very greatly impaired (distortions, high distortion factor etc.). To reduce the kink in the transmission characteristic during current transfer, the amplification properties--particularly in the range of the balanced state--must be matched to one another.